Wavelength conversion methods that use excitation light produced by solid-state light source such as laser diodes (LDs) or light emitting diodes (LEDs) and photoluminescent wavelength conversion materials such as phosphors or quantum dots can produce high brightness light at wavelengths different from the wavelength of the excitation light. In conventional devices, excitation light impinges on a wavelength conversion material, which absorbs the excitation light and emits light at a wavelength higher than the wavelength of the excitation light.
It is now common to implement white light sources, such as solid-state white light sources, using photoluminescent wavelength conversion materials. An LED diode that is operable to generate excitation light in the UV or blue region is used in conjunction with the excitation light source to generate, for example, white light. As taught in U.S. Pat. No. 5,998,925, white LED lighting systems include one or more photoluminescent materials (e.g., phosphor materials), which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emit(s) yellow light or a combination of green and red light, green and yellow light, green and orange light, or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white.
When operating using a direct current (DC) drive, a relatively continuous current level is maintained to drive the excitation light source (e.g., the LED). Therefore, for DC-based lighting applications, the photoluminescent materials used in the wavelength conversion components are preferred to have a decay time of less than a millisecond so that the light can be turned on and off immediately when the electrical switch is turned on and off, respectively.
However, it is possible that an alternating current (AC) source may be used to drive the LED lighting system. With AC power supplies, the electrical current forms a wave pattern that “alternates” to different current levels, e.g., usually a sine wave. An LED that is operable with AC currents is called an AC LED. A rectifier can be used to provide a double frequency of the input current to drive the AC LED.
The rectifier can be implemented as a simple rectified circuit without a capacitor or other more complex IC designs so as to avoid using electrolytic smoothing capacitors that have a lifetime much less than the expected lifetime of the LED. For other AC LEDs, the LEDs themselves, which are diodes, can be configured as a self-rectifying configuration—for example strings of serially connected LEDs which are connected in parallel with opposite polarities. Another example would be to provide the LEDs within the arms of a bridge rectifier configuration. In self-rectifying configurations one fraction of the LEDs will be operable on positive cycles of the drive current whilst another fraction will be operable on negative cycles of the drive current. As a result, in AC LEDs that use self-rectifying configurations the LEDs, and hence light output, will be modulated at the drive current frequency and the light output may appear to flicker. Moreover, whilst simple drivers without a capacitor have the advantage of lower cost and longer life time, such AC lighting systems will also have their light output modulated at the frequency of the input AC current, which will result in flickering.
Therefore, there is a need for a better approach for implementation of AC-based lighting that maintains the advantages of using cost-effective rectifier circuits, without unwanted AC-related artifacts such as flickering.